This invention relates to internal combustion spark ignition piston engines of the type employing a prechamber or auxiliary chamber adjacent each main combustion chamber, the chambers being connected by a torch opening. Relatively rich mixture is ignited in the auxiliary chamber at the end of the compression stroke, and this causes a flame to be projected into the main combustion chamber to ignite a relatively lean mixture. An engine of this general type is disclosed in the Date et al U.S. Pat. No. 3,830,205 granted Aug. 20, 1974.
Internal combustion engines of this type operate advantageously for reduction of the NO.sub.x content in the exhaust gases, and for greater reduction of NO.sub.x the lean mixture supplied to the main combustion chamber is so lean as to be within a range of unassured or impossible spark ignition. Prior to this invention, a single torch opening has been provided near one side of the main combustion chamber for projection of a flame across the chamber to the other side. This system has a disadvantage when the mixture is extremely lean because the rate of flame propagation is low. At the end of the compression stroke of the piston, particularly under operating conditions close to full open throttle where the effective compression ratio is great, the lean mixture located in a region far away from the torch opening is affected by the pressure rise caused by the ignition of the mixture in the neighborhood of the torch opening so as to be adiabatically compressed to a high temperature. It self-ignites to burn abnormally to develop knocking, before the torch flame reaches the region in question. This self-ignition is likely to occur at the end of the compression stroke in the neighborhood of the exhaust valve which is generally the hottest spot in the main combustion chamber.
In order to prevent the abnormal burning and possible knocking it is necessary for the torch flame to reach the region before self-ignition occurs. While this result may be accomplished by simply placing the single torch passage so that the direction of the flame is toward the exhaust valve, such an arrangement retards the ignition of the lean mixture in the direction of the main intake valve, with the result that the overall combustion in the main combustion chamber is poor. Furthermore, because the rate of flame propagation is very slow in the lean mixture, the single torch opening has been located as close to the exhaust valve as possible and near the center of the main combustion chamber. However, this retards the propagation of flame to portions of the lean mixture located remote from the exhaust valve, also bringing about poor combustion overall.
It is therefore an important object of this invention to provide a torch flame ignition system for a very lean mixture which minimizes self-ignition and knocking even when the engine is under full load. This object is achieved by providing a plurality of torch openings between the auxiliary combustion chamber and the main combustion chamber, and positioning them so that they direct torch flames toward both of the valves in the main combustion chamber as well as directing another torch flame toward a region of the main combustion chamber remote from said valves.
Other objects and advantages will appear hereinafter.